Viewed in visible light, the sky appears as a dark expanse, adorned with the twinkling lights of faraway stars. But with an X-ray telescope, the sky seems uniformly bright, bathed in a diffuse glow. For 37 years, astronomers have struggled to find the multitude of pointlike sources that combine to produce this impressionistic glow, known as the X-ray background.
Although they have made progress, the limited ability of telescopes to detect X rays in fine detail has hampered their efforts.
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